1. Field of the Invention
This invention relates to an improvement in rocket nozzles, and more particularly, to an improved method for forming an automatically extendible auxiliary nozzle member or exit cone for a rocket motor.
2. Description of the Prior Art
Deep space ballistic missile systems or satellite probes require high performance, low weight, and highly packageable primary propulsion systems. Excluding propellant tanks, the largest component of a propulsion system is the rocket exhaust nozzle. The rocket nozzle takes up a great deal of valuable space relative to its mass.
Conventional thrust nozzle exit cones for rocket motor ballistic systems are designed to provide the best average performance over the whole intended trajectory within the volumetric constraints created by launch tube clearance and interstage length limitations. One of the functions of the exit cone of a rocket nozzle is to provide an inclined surface against which the expanding exhaust plume of the rocket can bear, thereby providing some of the forward thrust of the rocket. The exhaust plume grows larger with increasing altitude of the rocket because of the decreasing pressure of the ambient atmosphere. As a result, in a conventional rocket nozzle, the exhaust plume at low altitudes is too small for the available surface of the exit cone. This allows the formation, inside the edges of the exit cone, of a partial vacuum which creates an atmospheric drag on the rocket. At high altitudes the exhaust plume is too large for the exit cone, and as a result, much of the potential energy is unused. A rocket nozzle that is sufficiently large to make full use of the expanding exhaust gases of a rocket in the low ambient pressures at high altitudes would normally occupy an inordinately large proportion of the available storage space in silos, submarines, aircraft, or between stages of a multi-stage missile.
Various proposals have been made in the prior art to provide a large expansion ratio nozzle that can be stowed in a collapsed or retracted configuration and thus made to fit into a minimal storage space, and that can be extended to the operating position after motor ignition. These have included the use of:
(a) an inflatable auxiliary rocket nozzle member or skirt that is inflated and extended by turbine exhaust gases when the rocket starts, as disclosed in U.S. Pat. No. 3,596,465 to Thomas O. Paine et al; PA1 (b) rocket nozzle members or exit cones that are highly flexible and can be compressed or expanded in various ways, as disclosed in said Paine et al patent, and in U.S. Pat. No. 3,346,186 to D. L. Fulton et al, in U.S. Pat. No. 3,358,933 to J. H. Altseimer, and in U.S. Pat. No. 4,272,956 to G. C. Lamere et al; PA1 (c) rocket nozzle members or exit cones wherein a thin sheet metal nozzle extension is inter-rolled into convolute form whereby an intermediate larger diameter portion encircles a small diameter mounting end portion, as disclosed in U.S. Pat. No. 3,711,027 to L. F. Carey, and U.S. Pat. No. 3,784,109 to J. W. Dueringer; and PA1 (d) rocket nozzle members or exit cones that are segmented longitudinally as disclosed in a paper entitled "Nested Extendible Exit Cone Solid Rocket Nozzle Engineering Evaluation Program" presented at the AIAA/SAE 14th Joint Propulsion Conference, Las Vegas, Nev., July 25-27, 1978, and as disclosed in copending application for U.S. Patent bearing Ser. No. 230,939, filed on Feb. 12, 1981 by Frank S. Inman, and assigned to Thiokol Corporation, the assignee of the present invention.
The Fulton et al patent shows an externally folded cloth or mesh nozzle extension on a rocket with the nozzle extension being deployed by pressurized telescopic tubes that are actuated when a predetermined altitude has been reached.
In FIGS. 1-4 of the Altseimer patent a corrugated rocket nozzle skirt is provided that is expandable transversely of the rocket motor longitudinal axis, the skirt being said to be made of metallic or non-metallic material, but not being foldable. At high altitudes where the gas pressure within the nozzle exceeds the ambient pressure, outward pressure expands the skirt. The nozzle area ratio thus is increased automatically as the atmospheric pressure decreases.
In FIG. 5 of the Altseimer patent an inwardly folded flared skirt is provided. That skirt is described as being made of a material such as reinforced rubber produced under the trademark GEN-GARD by the General Tire and Rubber Company or asbestos reinforced with inconel wire and impregnated with an ablative such as TEFLON, a tradmark of E. I. duPont de Nemours and Co., Inc. Firing of the rocket motor ejects the inwardly folded skirt, the ambient pressure acting against the pressure of the exhaust thereafter determining the degree of opening of the skirt.
The Carey and Dueringer patents disclose related subject matter, the material of the foldable rocket nozzle extension of the Carey patent being described as thin sheet metal but that of the Dueringer patent not being mentioned. In each of the patents a temporary cover member is attached to the exit end portion of the nozzle and seals the nozzle against internal gas pressures. Thus, when the rocket is fired, the cover causes the nozzle structure to unroll and deploy into extended configuration. The cover member is jettisoned after the nozzle is deployed.
In the Lamere patent, a rocket motor nozzle extension member or skirt is provided that is made of thin high temperature resistant material. The skirt is inwardly folded back in a pleated manner, deployment being automatically by combustion gases. When opened, the pleats are described as providing enough material to afford a frusto-concial surface in line with the expansion cone of the nozzle. Deployment of the skirt is delayed by an erodable barrier.
In the longitudinally segmented rocket nozzle exit cones of the paper presented at the AIAA/SAE 14th Joint Propulsion Conference and in the application for patent of Frank S. Inman, the various nozzle segments are actuated by separate mechanical means.
While concerned with the problem of how to make the auxiliary nozzle member or exit cone of a rocket nozzle more compact so as to fit in a minimal storage space, such proposals of the prior art are overly complicated or are "labor intensive", and hence expensive to build. The practice, for example, in fabricating the Lamere et al simple pleated auxiliary nozzle portion or skirt is for a skilled workman to take a flat ductile sheet, and using a die, repetitively and laboriously forming each pleat one by one. Following the formation of the pleats, the ends of the sheet are welded together. This is not only an expensive method of forming the skirt, being labor intensive, but it is difficult to insure that the resulting stresses and thicknesses of the pleats are uniform. As a consequence, the resulting form of the cone cannot be predicted with accuracy.
It is of significance that none of the aforementioned proposals of the prior art have disclosed, or otherwise taught or considered, how to make the rocket nozzle cone extendible or retractable, and at the same time, shown how to form the auxiliary nozzle member or skirt so as to insure that the stresses produced therein, and the thicknesses thereof are uniform to the end that when automatically deployed by the exhaust gases of the rocket, a more regular cone in line with the expansion cone of the nozzle is formed, and additionally, the form of the cone that results is one that can be predicted with accuracy.
Thus, there is a need and a demand for further improvements in automatically extendible auxiliary nozzle members or exit cones for rocket motors.